Niobium-gallium superconductor

ABSTRACT

A superconducting alloy having a Tc of &gt; OR = 20*K consists of a Beta -W alloy of Nb and Ga having the molar formula Nb3Ga. The alloy can be formed by annealing a Beta -W phase Nb-Ga alloy at 700*C.

United States Patent 1 Webb NIOBIUM-GALLIUM SUPERCONDUCTOR [75] Inventor: George Wendell Webb, Princeton,

21 Appl. 1%.: 224,000

[52] US. Cl 148/3, 75/174, 148/32, 148/133, 335/216 [51] Int. Cl. C22c 27/00, HOlf 1/04 [58] Field of Search 148/2, 3, 11.5, 32, 32.5, 148/133; 29/599; 75/174; 335/216 [56] References Cited UNITED STATES PATENTS 3,243,871 4/1966 Saur 29/599 3,256,118 6/1966 Speidel 148/2 OTHER PUBLICATIONS Naturwissenschaften, Vol. 54, N0. 18, 1967 pgs 489 &

[ Apr. 2, 1974 Zeitschrift fur Physik, 189, 1966, pages 401, 402 & 408-410.

Superconductors, Tanenbaum et al, New York, N.Y., 1962, Interscience Publishers, pages 42-45.

Russian Journal of Inorganic Chemistry Vol. 9, No. 9, Sept. 1964 pgs 1l721174.

Primary ExaminerCharles N. Lovell Attorney, Agent, or Firm-Glenn H. Bruestle; Birgit E. Morris [5 7] ABSTRACT A superconducting alloy having a T of 2 20K consists of a B-W alloy of Nb and Ga having the molar formula Nb Ga. The alloy can be formed by annealing a B-W phase Nb-Ga alloy at 700C.

4 Claims, 5 Drawing Figures 1 NIOBIUM-GALLIUM SUPERCONDUCTOR BACKGROUND OF THE INVENTION This invention relates to superconducting alloys and particularly to superconducting alloys of niobium and gallium.

Superconductivity in beta-tungsten (B-W) compounds of niobium and gallium was reported in 1958 by E. A. Wood et al., in Acta Crystallogr. I1, 604 (1958). The critical temperature (T of that compound, i.e., the temperature at which the onset of superconductivity was observed, was reported as being 14.5K. G. Meyer reported in Naturweiss 18, 489 I967), resistive transition onset of 19.85K for a Nb-Ga alloy comprising nominally 15 atomic percent Ga. This alloy, however, had a transition width of K and only a small percentage of the sample was superconducting at 19.0K. While this material may possibly have contained the alloy identified and claimed by the applicant herein, there is no way to know this as no separate phase as claimed herein was identified by Meyer.

In addition to the above, G. Otto reported transitions of 16K for Nb-Ga alloys having a nominal composition comprising 25 atomic percent Ga.

I have discovered a Nb-Ga alloy having a B-W structure and comprising about 25 atomic percentGa which has a superconductivity onset as high as 20.3K and having transition widths as low as ().7I(. This alloy is not stable above about 700C. It is this fact that probably accounts for the failure of others to observe this alloy in the past and which adds import to its method of preparation. The superconducting transitions reported by me were measured by the well-known inductive method.

SUMMARY OF THE INVENTION A superconducting alloy of Nb and Ga is characterized in that it has a B-tungsten structure, and a molar formula Nb Ga.

The novel superconducting alloy referred to above is prepared by annealing a B-tungsten-containing alloy of Nb-Ga at a temperature no greater than about 700C.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a photomicrograph of a multiphase alloy of Nb-Ga exhibiting both a B-W phase and a Nb Ga phase.

FIG. 2 is a plot of T onset as a function of the B-W lattice constant for specimens with well defined onsets and sharp X-ray diffraction patterns.

FIG. 3 is a graph showing the superconductive onset of a NbGa sample comprising a Nb Ga phase.

FIG. 4 is a graph showing the critical magnetic field as a function of temperature for Nb Ga, Nb Sn and Nb Al.

FIG. 5 gives the relevant portion of the Nb-Ga phase diagram and the T as a function of the temperature of the final anneal.

DETAILED DESCRIPTION OF THE INVENTION The novel superconducting alloy is a B-W alloy and contains about 25 atomic percent Ga. It may be represented by the formula Nb Ga. The novel Nb-Ga alloys may include other phases together with the Nb Ga and, in fact, usually do. For example, FIG. 1 is a photomicrograph of a Nb-Ga alloy at 1,000 x magnification. The nominal composition of the alloy is 32 atomic percent Ga and 68 atomic percentNb The specimen was formed by quenching the alloy from the melt at 1,850C to room temperature and subsequently annealing the quenched material for 47 hours at 700C. The resultant superconducting material was determined to consist of two phases, one phase, i.e., the B-W phase, is the dark dendritic structure on the photomicrograph and has an average width of 1.5 This phase has the molar formula Nb Ga and is characterized by the lattice parameter, a of from 5.163 5.167. The remainder of the sample is Nb Ga This sample had a superconductive onset T of 20.3I(.

The Nb Ga B-W phase of the novel superconducting alloys typically have a lattice constant of from about 5.163 to 5.168. The lower the lattice constant, generally, the higher the measured value of T FIG. 2 is a plot of T onset as a function of B-W lattice constant for specimens with both well defined onsets and sharp xray diffraction patterns. Error bars in dicate the range of lattice constants present in a given specimen as determined from the [622] X-ray diffraction line. Errors in the value of the mean of the lattice constant are estimated to be i 0.001 These latter errors are not shown in the plot. Points marked 0 are arcmelted specimens; those marked x are vapor deposited specimens and A and Clare values of specimens as recited in Wood et al., supra, and G. F. Otto, Z. fur Physik 215, 323 (1968). It can readily be seen from the plot that the lattice constant of the specimen is critical to a high T,. Generally, only specimens having a ,B-W phase with a mean value of a of less than about 5.168 exhibit values of T of 20.0K.

The typical narrow width of the superconductive onset of the novel alloys can be seen with reference to FIG. 3 showing the change in inductance as a function of temperature.

The novel superconductor, Nb Ga, with a mean lattice constant of 5 5.168 is superior to other binary superconductor materials not only in the fact that it can be made to have a higher T, than prior art binary superconductors, but also in the fact that higher magnetic fields can be applied to the novel superconductor. A comparison of the critical magnetic fields (H in kilogauss as a function of temperature in K for Nb Ga, Nb Sn and Nb Al may be seen with reference to FIG. 4. It is apparent from the figure that Nb Ga has a considerably higher H than these other binary alloys of niobium.

The fact that Nb-Ga alloys having a B-W phase of the composition Nb Ga and a lattice constant of 5.163 5.168 has not been found heretofore is probably due to the instability of this phase at temperatures above about 700C. In fact, this phase is formed by prolonged annealing of Nb-Ga alloys at 700C. It may also be formed by vapor phase deposition at 700C, however, the lattice constants of vapor phase grown specimens have not, to date, been as low as are grown and then annealed specimens. The vapor deposited specimens generally have values of T below that of arc grown material, i.e., T of about 19K. These materials are nevertheless potentially more useful than Nb Sn because of their high H The complexity of the Nb-Ga alloy system can be seen with relation to the phase diagram of FIG. 5. It is apparent that a pure B-W phase cannot be formed in a Nb-Ga alloy system when the specimen is prepared from the melt. When preparing the novel alloy from the melt, it is preferred to use a nominal composition of from about 20 to 33 atomic percent gallium and 67 80 atomic percent niobium. The pure materials are typically melted at a temperature of about 1,850C and the melt is quenched. The alloy thus formed is then annealed at 700C for extended periods of, for example, from 8 hours to several days. It can be seen from FIG. that annealing at temperatures of about 700C is important in obtaining high T. specimens. The characteristics of several specimens prepared from different nominal atomic percentages of gallium and heat treated under different conditions is given in Table 1.

It can be seen from the table below that only those specimens which were annealed at 700C or below (660C) exhibited a T onset of 20K. The specimens which were not annealed and the specimen an- 4 niobium and gallium comprising a B-tungsten phase characterized by a mean lattice constant of from 5.163 to 5.168 comprising the steps of:

a. forming a melt of gallium and niobium, said melt consisting of from to 33 atomic percent of gallium and from 67 to 80 atomic percent of niobium;

b. quenching said melt to form a solid; and c. annealing said solid at a temperature of from about 600C to about 700C for extended periods of time to form the desired p -tungsten phase alloy. 2. A superconducting alloy having a T onset of 2 20K comprising a B-tungsten phase having the molar formula Nb Ga and said alloy having an average lattice 5 constant of from 5.163 to 5.168.

3. The superconducting alloy recited in claim 2 wherein said alloy has a T onset of at least 20.3K

4. The superconducting alloy recited in claim 2 TABLE 1 Nominal High Temp. Low Temp. Inductance Change During fi- M1 03, X-Nb Ga Heat Anneal Transition a 7 c Comp Treatment r onset 20% 50% 80% (K) 21.5% 1870C 163 hrs 660C 20.1 19.9 19.8 19.7 5.1635.178 weak present 21.5% 1870C powdered 42 hrs 700C 20.15 19.8 19.4 14.7 5.l635.181 weak present 24% 1790C powdered 23 hrs 700C 20.3 18.1 15.9 15.6 5.1665.l76 weak absent 32% as are none 16.5 14.8 14.6 13.9 i absent melted 32% 1850C none 17.5 17.3 17.0 14.0 5.1685.177 10.312 5.067 absent 32% 1850C 16 hrs 700C 20.0 19.7 18.4 15.5 5.169-5.176 10.306 5.066 absent 32% 1850C 31 hrs 700C 20.3 20.1 19.8 16.5 5.164-5.168 10.312 5.068 absent 32% 1850C 47 hrs 700 C 20.3 20.0 19.6 15.9 5.163-3.167 10.307 5.067 absent 32% 1850C 62 hrs 950C 15.9 13.4 12.6 11.0 5.172-5.179 10.310 5.065 absent 37% as are none 2 6 absent 10.306 5.066 absent melted wherein the midpoint of transition is not less than 19.6K. 

2. A superconducting alloy having a Tc onset of > or = 20*K comprising a Beta -tungsten phase having the molar formula Nb3Ga and said alloy having an average lattice constant of from 5.163 to 5.168.
 3. The superconducting alloy recited in claim 2 wherein said alloy has a Tc onset of at least 20.3*K.
 4. The superconducting alloy recited in claim 2 wherein the midpoint of transItion is not less than 19.6*K. 